1. Field of the Invention
This invention relates to a stencil mask used in semiconductor processes and a method of manufacturing the stencil mask.
2. Description of the Related Art
Semiconductor device manufacturing processes include the process of forming a plurality of metal oxide semiconductor field-effect transistors (MOSFETs) differing in the conductivity type of channels in a substrate or the process of forming a plurality of MOSFETs differing in threshold voltage. In the process, when impurity ions are implanted into wells, channels, or polysilicon layers, a stencil mask having openings in it is used. The stencil mask is provided a specific distance above the semiconductor substrate. Impurity ions are implanted into a specific region through the stencil mask.
A stencil mask is used to irradiate particles or electromagnetic waves onto a substrate to be processed. The particles include charged particles, such as electrons or ions, and neutral particles, such as atoms, molecules, or neutrons. Electromagnetic waves include light and X rays.
A stencil mask in semiconductor processes is generally formed out of an silicon-on-insulator (SOI) substrate 100 in the processes shown in FIGS. 10A to 10D. Hereinafter, the processes of manufacturing a stencil mask will be explained.
FIG. 10A shows an ordinary SOI substrate 100. The SOI substrate 100 is formed by, for example, implanting oxygen ions into a silicon substrate 101 and then annealing the resulting substrate at a high temperature. A silicon oxide film 102 is formed at a depth of several tens to several hundreds of nanometers from the top surface of the silicon substrate 101. On the silicon oxide film 102, a silicon thin film 103 is formed.
Next, as shown in FIG. 10B, a resist (not shown) is applied to the surface of the silicon thin film 103. The resist is processed by lithographic techniques, thereby forming a resist pattern. Thereafter, with the resist pattern as a mask, the silicon thin film 103 is etched anisotropically until the silicon oxide film 102 is exposed. After openings 104 are made in the silicon thin film 103, the resist pattern is removed.
Next, as shown in FIG. 10C, a resist (not shown) is applied to the back of the silicon substrate 101. The resist is processed by lithographic techniques, thereby forming a resist pattern. Thereafter, the silicon substrate 101 is etched isotropically with a chemical liquid, such as KOH. Specifically, the part where no resist pattern is formed on the silicon substrate 101 is etched isotropically until the silicon oxide film 102 is exposed, thereby forming a support 105. Thereafter, the resist pattern is removed.
Next, as shown in FIG. 10D, the silicon oxide film 102 exposed in the process of FIG. 10C is processed from its back with chemical liquid, such as fluoric acid, thereby removing the silicon oxide film 102 and exposing the silicon thin film 103. In this way, a stencil mask 105 with the openings 104 in it is formed.
As shown in FIGS. 11A and 11B, in the processes of manufacturing a semiconductor device, when impurity ions are implanted into a semiconductor substrate 106 to be processed, the stencil mask in which the openings 104 have been made is used.
As shown in FIG. 11A, above the ion implantation regions 107 of the semiconductor substrate 106, the stencil mask 105 is provided so that the openings 104 in the stencil mask 105 may face the region 107.
Next, as shown in FIG. 11B, impurity ions 108 are implanted from above the stencil mask 105. The impurity ions 108 pass through the openings 104 in the stencil mask 105 and are implanted into the ion implantation regions 107 of the semiconductor substrate 106. Since there is no opening 104 in the non-implantation region, impurity ions 108 are cut off by the stencil mask 105. In this way, the stencil mask 105 cuts off ions repeatedly, allowing the cut-off ions to be accumulated, which gives rise to a charge-up problem.
The stencil mask 105 is composed of the silicon thin film 103 in which an opening pattern is formed, a support 101 that supports the silicon thin film 103, and a silicon oxide film 102, an insulating film, between the silicon thin film 103 and the support 101. Therefore, the electrical conductivity of the stencil mask 105 is low, which causes the amount of charge accumulated in the stencil mask to increase.
Charged particles are implanted from a charged particle implanting source above the stencil mask through the openings in the stencil mask into the semiconductor substrate. When the stencil mask is charged up, however, the charges accumulated in the stencil mask bend the trajectories of the charged particles implanted vertically from above. The changes in the trajectories cause the charged particles to be implanted into the semiconductor substrate in such a manner they deviate from the predetermined implantation region of the semiconductor substrate.
Furthermore, bringing the charged-up stencil mask closer to the semiconductor substrate causes the silicon thin film of the stencil mask to be deformed by electrostatic force, which is a problem. To avoid the effect of such charge-up on the stencil mask, the following configurations have been developed.
A first method is to cover the surface of a stencil mask with a metal film whose electrical conductivity is high. This configuration has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 6-244091. Since in the method, the stencil mask is covered with a metal film whose electrical conductivity is high, the charged-up charges are allowed to escape in a short time and therefore the amount of the accumulation is low. Therefore, it is possible to prevent the trajectories of the charged particles implanted from being bent by the accumulated charges.
A second method is to provide a conductive material film whose electrical conductivity is high in place of the insulating film formed between the silicon thin film and the support. This method has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-216613.
In the first method, however, when charged particles are implanted, the metal film covering the stencil mask is sputtered due to the collision of charged particles, with the result that the semiconductor substrate to be processed can be contaminated.
Furthermore, in the process of forming a metal film on the stencil mask, the metal film also adheres to the sidewall of the opening pattern formed in the silicon thin film part. As a result, the metal film formed on one side or both sides of the silicon thin film projects into the openings, causing the problem of narrowing the opening pattern in the silicon thin film part.
On the other hand, the second method requires a plurality of processes to form a conductive material film between the silicon thin film part and the support. Consequently, the stencil mask manufacturing processes become complicated, which leads to an increase in the manufacturing cost. Therefore, there have been demands for a stencil mask capable of suppressing the contamination of a semiconductor substrate and reducing charge-up.